This invention relates to a storage process for high resolution images on a medium of the video disk type, and to the encoders and decoders designed for the implementation of said process.
The present invention is concerned more particularly with a process for recording and restituting high resolution images decomposed in several partial images, each partial image capable of being analyzed according to television standards.
A known method for the storage of fixed high or very high definition images, i.e., is with more than one thousand lines per image, is the use of a standard type video disk intended to store or to restitute images having a 525 or 625 line resolution according to current television standards.
According to a technique described by the inventors, more particularly in the work "videodisks" edited in 1986 by the Publishers MASSON, the high resolution image is divided into several partial low resolution images, each recorded on one turn of a video disk.
To switch over to a 1249 line standard, the said high resolution image is divided in four partial 626 line images.
A device inserted between the output of the video disk reader and a high resolution monitor reconstructs a high resolution image from four partial images.
The strict recomposition of partial images is difficult by reason of the persistence of a joining area in the form of a vertical band a few pixels wide. More particularly in the case of a high resolution image having areas of uniform illumination, characterized by a strictly constant luminance and chrominance on either side of the vertical frontier between two partial images, it is nearly impossible to maintain a constant luminance level throughout the length of the lines.
In fact, by reason of the transient states, significant of the discontinuities which show the beginning and the end of each line of a television image, the corresponding signal features an exponential growth or decay whose time constant is equal to the reciprocal of the video amplifier band-pass.
Moreover, in the case of a series of white lines, the enable time of the significant steady state further increases the width of the luminance disturbance zone.
When the resulting effect affects the right and left screen ends, it is not perceptible by the televiewer but it is unacceptable if it affects the middle of the image, all the more in the case of high resolution images which are supposed to meet high quality requirements.
The purpose of the present invention is to remedy this disadvantage.